1. Field
This disclosure relates generally to the field of pressure sensors, and more specifically, to cavity-style pressure sensing devices subject to rapid decompression events.
2. Related Art
Due to the significant influence tire pressure has on vehicle safety and efficiency, tire pressure monitoring systems have been mandated for light motor vehicles to help alert drivers to severe under inflation events. Two basic types of tire pressure monitoring systems (TPMS) have been adopted to satisfy this mandated requirement: direct and indirect systems. Direct TPMS employs pressure sensors on each tire to physically measure tire pressure and report that information to monitoring devices in the vehicle. Such systems can identify under-inflation situations, simultaneous or singly, in any combination. Such TPMS configurations can display real-time tire pressures that each location monitored, whether the vehicle is moving or standing still. Indirect TPMS does not use physical pressure sensors, but instead infers tire pressure by monitoring information such as individual wheel rotational speeds. Indirect TPMS operates under the presumption that an under-inflated tire's slightly smaller diameter results in a higher angular velocity for that tire. Additional signal processing techniques can be used for indirect TPMS to determine simultaneous under inflation and load shifting.
Typically, pressure sensors used in direct-sensor TPMS are installed inside the tires. Such internal sensors are typically either fitted to the wheel rim or two and in-tire section of the valve stem when the tire is fitted. Such sensors utilize radio frequency (RF) communication to transmit pressure readings and other data collected from the sensors.
The internal pressure sensors are typically semiconductor devices mounted in a cavity-style package. Typical cavity-style pressure sensing devices are placed in a pre-molded enclosure on a lead frame and the cavity is filled with silicone gel. But micro gaps can be present at interfaces between the plastic molding material and the metal contacts of the molded package. When such a TPMS device is exposed to high pressure (e.g., during tire inflation), high-pressure air can diffuse through the silicone gel and be stored in the micro gaps. If a rapid decompression event occurs, the air stored in the gaps can escape, resulting in bubbles forming within the silicone gel. The bubbles can interact with wirebond connections and microelectromechanical systems (MEMS) structures, which can cause errors in pressure readings or device failure. It is therefore desired to provide a pressure sensor package that does not have a tendency to develop micro gaps so that bubble formation can be avoided.
The use of the same reference symbols in different drawings indicates identical items unless otherwise noted. The Figures are not necessarily drawn to scale.